voltage dropping
 

Here is my problem. When I drive the car the voltage drops and drops until eventually the car will just shut down due to not enough power. I first thought it might be battery or alternator. I replace the battery but problem still happens, but if I get the battery recharged I can drive the car for another hour or so until the voltage drops enough. I had the alternator tested at two stores both say it is working great.

My question is what things should I start doing now to fix this problem. Any advice other than lighting the car on fire and collecting insurance money is welcomed ;) Thank you.

Check to make sure all of your wiring is correct to the alternator and all attached.
Did you do anything recently to the car and then the problem started?

Well if your battery is new and your alternator is new (and verified good), you likely have a cable issue - double check them! Are they tight?

Sorry I am repeating what Doc said

I had the SAME EXACT problem. Check your fuses in the fuse box right by your left leg when sitting in the driver's seat of the car. I had blown a 20 amp fuse (prob replace all of them if they are old, that's what I did). I replaced that and it completed the circuit, allowing the alternator to charge the battery.


Good luck!

Ryan

Ok a bit of an update. I think I may have found something. If I run a voltmeter between the negative battery terminal and negative battery cable I get a reading of 10 volts! I am guessing this can't be right. I tried removing each fuse one at a time under the dash but each time the reading was still 10 volts. What else could be drawing 10 volts! Could this be causing the symptoms I am seeing? Thanks again for all the help.

You just have to clean your battery terminals and the IDs of the cable clamps. They are corroded. Go to the autostore and buy one of those plastic 2 piece gizmos that have a cylindrical external wire brush for the clamp ID and a corresponding internal wirebrush for the battery terminal OD. While you're checking out get a small packet of anti-corrosion gel that they always have in a rack at the checkout counter for about a buck. The 2 piece wirebrush gizmo is about $4 in plastic and about $10 in metal. The cheap one is the best one.

In a pinch the Home Depot near you has a similar tool for abrading copper water tubes to prepare for soldering.

While at the autostore you might want to pickup a box of assorted fuses for spares and put them in your glovebox. At home, if you're really ambitious, you might want to test the fuses in the fusebox near your left knee and ascertain what purpose each of them is for because sometime s the fusebox diagram is inaccurate, for example, if the cover was replaced by an incorrect one! I check those fuses and make a little note in my palm in case I ever need it (which i haven't). I make a special note of which fuse controls the fuelpump so that if I ever have to park the car where I fear vandals or thieves might be a problem , like in longterm airport parking, I can quickly remove that fuse until I return and put it back in.

People waste a lot of money replacing OK parts, especially batterys and alternators. Mostly because of ignorant kibitzers. In 50 years of owning cars I've replaced one battery because a mechanic told me it was dead, and then I still hadn't solved the problem. But I've been told at least 20 times that I had to replace my battery and it was not true, it was simply corrosion, just like your problem. But I'm still kicking myself for that one time I wasted $40 on a new battery I didn't need.

He's saying he has a 10 volt drain on the battery...

Circuits don't 'draw' voltage, they draw current from the battery and driven (pushed, as it were) by the battery voltage.

"If I run a voltmeter between the negative battery terminal and negative battery cable I get a reading of 10 volts! "

Good test! You did the right thing, and you have isolated the problem precisely. This reading should be 0 or close to it.

The problem is NOT that you have a high current drain someplace, but just the opposite: the corrosion between the post and the clamp is so bad that no electricity can get through. Voltage and current are complementary things: 'voltage' is potential energy: it sits there ready to go when you close a switch to power something on. But otherwise it does nothing but sit and wait. 'Current' is what flows thru the wire when you close that switch. To get useful 'power' you need both: the potential energy and the current flowing through the wire.

When you close the switch then the battery voltage causes a current flow through the circuit, but right away there is so much resistance in the corrosion that just a tiny bit of current can get thru the nasty corrosion and then the voltage drops by 10 volts right there and only about 2 or 3 volts is available to the starter, and a starter won't work (nor will hardly anything else in a car).

There is a strict law about voltages around a circular circuit: all the voltages in the series circuit (like a daisy chain, or the serial christmas tree lights) must add up to the supply voltage of the battery. thus, when you have 10 volts across the corrosion then that leaves only about 2-3 volts to power the auto electrics. Not enough.

I've seen cars with as little as 1.5 volts drop, where you measured 10, that would not spin the starter. That's with pretty good voltage available at the starter.

If you live in an ultra-cold climate you can encounter another source of high resistance in the circuits: the aluminum battery cables used in many cars of that era. They conduct poorly in cold weather. But they're not too hard to replace with copper cables from the autostore.

You can shoot most low voltage bugs with your voltmeter. You might also want to get a battery electrolyte tester to see if your battery is good. I bought a little battery hydrometer for about $4 several years ago that is about 4" long and draws a little battery fluid in and then floats 1 to 4 colored balls to indicate the gravity (and thus the ion state) of the cell.

A tip! When you loosen the clamps to clean them, remove the negative terminal first. That way if your wrench handle accidently touches chassis metal when removing the positive clamp you won't get a shower of sparks! Those sparks MAY ignite the hydrogen gas that sometimes accumulates around a wetcell. Friend of mine had one explode and blast him halfway across a gas station once when he did that. When I attach a cable to the positive terminal I always duck my head below the fender line to do it.

It's all just household science, you know. I little knowledge and thinking logically about things can save your life.

For example, here's something that if you think about it can save your life one day on the freeway: you're driving on a clear freeway and see black skidmarks from a panic stop. What can you learn from studying the skidmarks as you drive by that will make a slight change in your driving technique that may save your life someday?

Did you remove and test the fuses to make sure they were good? Just removing a couple fuses will not fix the problem I think you have. It's not that expensive to replace all the fuses in the fuse box...

I was getting the same screwed up readings from my battery terminal and what not.
JUST REPLACE YOUR FUSES OR CHECK THEM TO MAKE SURE THEY ARE GOOD.

Pull up the wiring harness diagram. There is a 20 amp fuse that connects the circuit with the alternator and battery, etc. If this is bad, the circuit will not complete, resulting what in you just described.

Ryan

RX7vert:

Is your problem fixed?

I had the same type of problem. I would charge up my battery and it would die overnight. The alternator was pushing out 14.4V as well. What was wrong was the reverse drain protection diodes in the alternator's voltage regulator were hooped and resulted in a current leak though the regulator when the car was off. IE, current was flowing though the large cable of the alternator to ground, thus draining the battery.

Try this:

-turn off car and disconnect positive battery terminal.
-turn mulitmeter into current measurement (if not autoranging choose largest scale.) make sure your probe lead is in the current position, not the voltage.

-place one lead on battery positive, other lead on battery positive cable. You are making a loop from the positive of the battery, thru the multimeter to the car's positive.

-Check for a reading.. if there is nothing shown, keep going down on the scale. You should see a small drain from the clock and stereo, but not a large one. If you see something like an amp, then there is a current drain.

-try disconnecting lines to alternator and see if your current drain stops.

Do not do this with the car running. Most multimeters will not handle the 40amps your car pulls while running.

also, If you are measuring the volt drop of 10V from the negative battery cable to the negative terminal with the car off, you have a problem.

Since the car is off, you should not have a potential of 10V at the negative cable. If there is a 10V potential at the negative terminal to the negative cable with the car off and the cables disconnected, then you have a short to ground somewhere.

Hate to correct you but I have to. Voltage is a measure of electrical pressure, amperage is a measure of electrical current flow.

If you hooked a potentiometer (voltmeter) between the battery negative and the negative cable and you had a reading of 10 volts dropped you have a problem with your connections or cables.

Wish I had the car infront of me because I could give you a much better diagnosis if it was the case.

What I do recommend though is to perform a parasitic draw test. Unhook the battery positive, and hook a amnmeter up in series and see how much current you are drawing will all electrical loads off. Should be under 0.1A.

Also I would check all your cables and check all of your fuses (with the voltmeter) testing across the terminals making sure there is a potential difference of zero (or a very small number) across the fuses.

I'm off my nut on this. Since I can't edit it out.....

You should see maybe 70mA flow if you connect your multimeter in series with the battery. If you see any more than this, then you have a short to ground. Install a new stereo or amp recently?